Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically with Storeatula major (Cryptophyceae) as prey. G. galatheanum , like many photosynthetic dinoflagellates, contains high amounts of n-3 long-chain-polyunsaturated fatty acids (LC-PUFA) such as docosahexaenoic acid (DHA, 22:6n-3) and the hemolytic toxic fatty acid 18:5n-3. We hypothesize that a benefit of phagotrophy in G. galatheanum is the acquisition of precursor linolenic acid (18:3n-3) that fuels

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... -PUFA synthesis. Phototrophs grew at 0.37 d Ϫ 1 , while mixotrophs grew at 0.40 d Ϫ 1 with a feeding rate of 0.62 d Ϫ 1 . Photosynthesis was lower in mixotrophs (3.7 pg C cell Ϫ 1 h Ϫ 1 ) than phototrophs (4.9 pg C cell Ϫ 1 h Ϫ 1 ). DHA levels were higher in mixotrophs [3.7 ( ϩ / Ϫ 0.11) pg cell Ϫ 1 ] than phototrophs [3.0 ( ϩ / Ϫ 0.16) pg cell Ϫ 1 ] and prey [0.4 ( ϩ / Ϫ 0.01) pg cell Ϫ 1 ]. 18:5n-3 levels [1.7 ( ϩ / Ϫ 0.03) pg cell Ϫ 1 ] were similar in phototrophs and mixotrophs. An intermediate in n-3 LC-PUFA synthesis, 20:4n-3, accumulated in mixotrophs [0.6 ( ϩ / Ϫ 0.27) pg cell Ϫ 1 ] relative to phototrophs (not detected) and prey [0.03 ( ϩ / Ϫ 0.002) pg cell Ϫ 1 ]. Low ratios of linolenic acid to DHA in phototrophic G. galatheanum (0.14) relative to mixotrophic G. galatheanum (0.29) and prey (2.14) are consistent with substrate limitation of LC-PUFA synthesis in phototrophs. Accumulation of 20:4n-3 suggests incomplete conversion of linolenic acid to DHA, possibly due to conditions in batch culture. We conclude that precursors for n-3 LC-PUFA biosynthesis in G. galatheanum may be acquired through ingestion of S. major , and may partially control feeding/photosynthesis in mixotrophic populations.